The crystallization and morphology of polyethylene and its blends/

The techniques of neutron and x-ray scattering have been used to study the morphology and crystallization behavior of polyethylene and blends of polyethylene. Synchrotron radiation was used to study the crystallization behavior of blends of high density polyethylene/ low density polyethylene (HDPE/LDPE) and linear low density/ low density polyethylene (LLDPE/LDPE). Simultaneous real time small and wide angle scattering from blends slowly cooled at (0.5\sp\circC/min) seem to indicate that the lamellae are formed in bundles of primarily one component. For blends quickly cooled from the melt (quenched to 60\sp\circC) on the other hand, the lamellae are randomly mixed together. HDPE/LDPE and LLDPE/LDPE blends show qualitatively the same crystallization behavior throughout the composition range except for 10%/90% LLDPE/LDPE. At this composition, extensive cocrystallization may be occurring in even slowly cooled samples. Small angle neutron and x-ray scattering was used to determine the location of the short chain branches in selectively deuterated LLDPE. Specially prepared LLDPE with the main chain deuterated was used in these experiments to provide contrast for neutron scattering. Despite density contributions to the neutron scattering from crystalline and amorphous regions, differences between the x-ray and neutron scattering suggest that the concentration of branches may be enhanced at the crystal- amorphous boundary. The extent of this branch-rich region was estimated to be about 30A. Lastly, the chain orientation of ultra high molecular weight PE (UHMWPE) was examined by small angle neutron scattering. A circularly averaging technique was applied in order to avoid sample alignment problems. Between extension ratios of 12 and 60, hot drawn (125\sp\circC) gel crystallized UHMWPE does not show appreciable change in the perpendicular radius of gyration. However, changes in the asymptotic behavior of the scattering intensity from I $\sim$ q\sp{-1.56} at 12x to I $\sim$ q\sp{-1.2} at 60x indicate a change in geometry toward more rod like segments in the higher drawn material

Pathways to Macroscale Order in Nanostructured Block Copolymers

Polymeric materials undergo dramatic changes in orientational order in response to dynamic processes, such as flow. Their rich cascade of dynamics presents opportunities to create and combine distinct alignments of polymeric nanostructures through processing. In situ rheo-optical measurements complemented by ex situ x-ray scattering reveal the physics of three different trajectories to macroscopic alignment of lamellar diblock copolymers during oscillatory shearing. At the highest frequencies, symmetry arguments explain the transient development of a bimodal texture en route to the alignment of layers parallel to the planes of shear. At lower frequencies, larger-scale relaxations introduce rearrangements out of the deformation plane that permit the formation of lamellae perpendicular to the shear plane. These explain the change in the character of the pathway to parallel alignment and the emergence of perpendicular alignment as the frequency decreases

Dynamics of Shear-Induced Alignment of a Lamellar Diblock: A Rheo-optical, Electron Microscopy, and X-ray Scattering Study

In-situ rheo-optical methods are used to guide electron microscopy (TEM) and X-ray scattering (SAXS) studies of structure development during flow-induced alignment in a lamellar block copolymer melt (nearly symmetric polystyrene−polyisoprene diblock, ODT ≃ 172 °C). The progress of shear-induced alignment is recorded in real-time using flow birefringence; at selected points during alignment samples are taken for ex-situ characterization by TEM and SAXS along all three axes (v, ∇v, ∇ × v) of the flow geometry. Three different trajectories are examined:  perpendicular alignment and two qualitatively different routes to parallel alignment in the high-frequency regime (ω > ω'_c). In general, the initial “fast” process not only enhances the projection of the orientation distribution that corresponds to the final state but also increases other projections of the distribution; the late-stage “slow” process eliminates these other projections and perfects a single alignment. For example, the highest frequency path to parallel alignment begins by transforming poorly organized regions into layers that are predominantly oriented along the parallel and transverse directions. The transition to the slow process is marked by the development of a characteristic texture in which tilt wall boundaries normal to the flow direction separate bands that form a repeating “chevron” pattern (layers tilted up and down about the ∇×v axis). The coarsening of this pattern dominates the slow process, during which the transverse projection is also eliminated

Synchrotron X-ray scattering studies on the structural evolution of microbial poly(3-hydroxy-butyrate)

The crystallization behavior of microbially synthesized poly(3-hydroxybutyrate) was studied in detail using time-resolved small-angle X-ray scattering. This polyester was found to undergo primary crystallization as well as secondary crystallization. In the primary crystallization, the thicknesses of the lamellar crystals were sensitive to the crystallization temperature, but no thickening was observed throughout the entire crystallization at a given temperature. The thickness of the lamellar crystals in the polyester was always larger than that of the amorphous layers. Secondary crystallization favorably occurred during the later stage of isothermal crystallization in competition with the continuous primary crystallization, forming secondary crystals in amorphous regions, in particular in the amorphous layers between the primarily formed lamellar crystal stacks. Compared to the primarily formed lamellar crystals, the secondary crystals had short-range-ordered structures of smaller size, a broader size distribution, and a lower electron density.open1113sciescopu

Structural evolution in microbial polyesters

The crystallization behavior of microbially synthesized poly(3-hydroxybutyrate) (PHB) and its copolymers [P(HB-co-HHx)] containing 2.5, 3.4, and 12 mol % 3-hydroxyhexanoate (HHx) comonomer and the melting of the resultant crystals were studied in detail using time-resolved small-angle X-ray scattering and differential scanning calorimetry. The polyesters were found to undergo primary crystallization as well as secondary crystallization. In-the primary crystallization, the thicknesses of the lamellar crystals were sensitive to the crystallization temperature, but no thickening was observed throughout the entire crystallization at a given temperature. The thickness of the lamellar crystals in the PHB homopolymer was always larger than that of the amorphous layers. In the copolymers, by contrast, the randomly distributed HHx comonomer units were found to be excluded from the lamellar crystals into the amorphous regions during the isothermal crystallization process. This interrupted the crystallization of the copolymer chains, resulting in the formation of lamellar crystals with thicknesses smaller than those of the amorphous layers. The lamellar crystals in the copolymers had lower electron densities compared to those formed in the PHB homopolymer. On the other hand, secondary crystallization favorably occurred during the later stage of isothermal crystallization in competition with the continuous primary crystallization, forming secondary crystals in amorphous regions, in particular in the amorphous layers between the primarily formed lamellar crystal stacks. Compared to the primarily formed lamellar crystals, the secondary crystals had short-range-ordered structures of smaller size, a broader size distribution, and a lower electron density.X113531sciescopu